Power electronics in motor drives: Where is it? Nagarajan Sridhar Product Marketing Manager High Performance Isolated Power Solutions Power Management Texas Instruments
Motor drives have an undeniable presence in key systems used in our daily lives. As such, energy savings through efficiency and reliability improvement is of paramount importance and is the key focus for suppliers and regulatory standards bodies. Everyday we see systems in motion all around us. What makes them move? On the outset, it may be due to wheels as in the case of an automobile. What actually drives these movements, though, are motors. Additionally, many household appliances such refrigerators, air-conditioners, ventilation fans, washers, driers and so many others all require electric motors. One can see that motors are part of our day-to-day life. The goal of this paper is to discuss the role of power electronics – the various components and requirements – in motor drives through applications that we use and encounter in household and industrial environments. What is a motor drive?
The function of the motor drive is to draw electrical energy from the electrical source and supply
An electric motor is a device that converts electrical
electrical energy to the motor, such that the desired
energy to mechanical energy. It also can be viewed
mechanical output is achieved. Typically, this is the
as a device that transfers energy from an electrical
speed of the motor, torque, and the position of the
source to a mechanical load. The system in which
motor shaft. Figure 1 shows the block diagram of a
the motor is located and makes it spin is called the
motor drive.
drive, also referred to as the electric drive or motor drive.
Motor Drive Motor Electrical Source
Mechanical Load
Power Converter
Controller
Sensor
Figure 1. Block diagram of a motor drive system
Power electronics in motor drives: Where is it?
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June 2015
The functions of the power converter circuit in the
and carbon emissions on the environment, various
motor drive are:
regulations across many countries have put forth and are continually working on governmental
• Transfer electrical energy from a source that could be of a given voltage, current at a certain frequency and phase as
mandates to improve motor drive efficiency.
the input
All these requirements make it compelling to have an efficient power converter system using switched-
• To an electrical output of desired voltage, current,
mode power supplies (SMPS). The SMPS uses
frequency and phase to the motor such that the required
semiconductor power switches (also called power
mechanical output of the motor is achieved to drive the
electronic switches) in a switch mode and on and
load
off states only, that yields 100 percent efficiency
• Controller regulates energy flow through feedback coming
in an ideal situation. Power electronics systems
from the sensor block
are primarily designed using silicon-based power
• Signals measured by sensors from the motor are
management with power semiconductor switches. These switches are power MOSFETs, bipolar
low-power, which are then sent to the controller
junction transistors (BJTs), and isolated gate bipolar
• Controller tells the converter what it needs to be doing. A
transistors (IGBTs) that have made significant
closed-loop feedback system is the method of comparing
improvements in their performances. Examples
what is actually happening to what the motor should
include lower on-state resistance, increased
be outputting, then adjusting the output accordingly to
blocking voltage, and higher drive currents.
maintain the target output
Furthermore, a lot of development is taking place using wide-band-gap semiconductors such as
Motor drive efficiency
silicon carbide (SiC). SiC is of particular interest to
Electric motors represent 45 percent of all electrical
motor drives that transfer very high power at high-
energy consumption across all applications.
voltage levels.
Increasing the efficiency of motor-drive systems could potentially result in a significant reduction in
Motor drive classifications
global electricity consumption [1]. With increasing
Before we delve into motor drive applications and
demand of electricity along with industrialization and
the role of power electronics in these systems,
urbanization across the globe, the ability to supply
here is a quick overview on how motor drives are
energy is becoming even more challenging. As part
classified (Figure 2).
of a global effort to reduce energy consumption
Electrical Motors
DC Motors
Brushed Motors
AC Motors
Brushless Motors
Induction Motors
PMSM* Motors
Figure 2. Classification of motors *PMSM = permanent magnet synchronous motors Power electronics in motor drives: Where is it?
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June 2015
Motor Type
Voltage Levels
Power Levels
Applications
Advantages
Disadvantages
Brushed DC
<100V
<100W
Toys, coffee machine, gate openers, etc.
Easy to spin, low cost
Brushes wear out, Inefficient
Brushless DC
<600V
Up to a few kW
Household appliances, white goods, pumps
Long life/reliable, high efficiency
Cost, complicated control
AC induction
>600V
>750 W
Industrial and factory automation
Low cost, less maintenance, rugged, reliable in wide power range
Starting issues, lowpower factor correction, complicated speed control
Table 1. Comparative analysis of motors
Table 1 summarizes where AC (induction) and DC
the DC back to AC into the motor using complex
(brushed and brushless) motors are used in terms
control algorithms based on load demand.
of voltage and power levels, along with the pros and
Figure 3 shows a block diagram of an AC motor
cons of each.
drive. The power stage and power supplies are
Power converter in motor drives
marked in teal.
The drive configuration for motors summarized
Power stage
in Table 1 are generally the same. However, what
The power converter topology used in the power
differs is the power converter topology in the
stage is that of a three-phase inverter which
power converter circuit. Since the bulk of these
transfers power in the range of kW to MW. Inverters
applications are moving towards brushless DC
convert DC to AC power. Typical DC bus voltage
(BLDC) or induction motors, our focus will be on
levels are 600-1200V. Considering the high power
applications that use these two types of motors.
and voltage levels, the three-phase inverter uses six isolated gate drivers (Figure 3). Each phase uses
In general, selecting a motor drive may require
a high-side and low-side insulated gate bipolar
looking at the power and voltage levels while
transistor (IGBT) switch. Operating usually in the
addressing questions that depend on the
20-30 kHz range, each phase applies positive
application. Examples could be the starting torque,
and negative high-voltage DC pulses to the motor
load inertia, pattern of operation, environmental
windings in an alternating mode. High-power
conditions, or the motor’s ability to regenerate.
IGBT requires isolated gate drivers to control their
Addressing these questions is outside the scope of
operations. Each IGBT is driven by a single isolated
this paper.
gate driver. The isolation is galvanic between the high-voltage output of the gate driver and the
AC motor drives
low-voltage control inputs that come from the
The AC motor drive, as the name suggests,
controller. The emitter of the top IGBT floats, which
requires an AC input to the induction motor used
necessitates using an isolated gate-driver. In order
to drive large industrial loads such as HVAC for
to isolate a high-voltage circuit with that of a low-
commercial buildings – pumps and compressors,
voltage control circuit, isolated gate-drivers are used
factory automation, industrial equipment that
to control the bottom IGBTs.
requires provisions for speed adjustments such as conveyor belts, tunnel boring, mining, paper mills,
Gate drivers convert the pulse-width modulation
and many others. An AC motor drive takes an AC
(PWM) signals from the controller into gate pulses
energy source, rectifies it to a DC bus voltage and,
for the FETs or IGBTs. Moreover, these gate drivers
implementing complex control algorithms, inverts
need to have integrated protection features such
Power electronics in motor drives: Where is it?
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June 2015
24VDC System Power Bus Off-line Power Supply
Motor Drive Safety MCU
AC/DC
Communication Interface
(with Fieldbus Integration)
(optional)
DC/DC Converters & LDOs
MCU
Isolated Gate Drivers
Control MCU
Industrial Ethernet PHY Industrial 485 PHY
Isolated DC/DC
Power Stage
OPA
Internal ADCs
Industrial Ethernet PHY
Industrial CAN PHY
PROT
Control
Communication
Industrial 485 PHY
100-1200V Line Power
VREF PRU
IGBTs
Sensors
Phase/Voltage Feedback (options) OPA
SAR ADC
AMP
Industrial CAN PHY
SDM Encoder
Position Feedback
Figure 3. Block diagram of an AC motor drive
as desaturation, active Miller clamping, soft turn-off
power MOSFET, showing similar current rise and
and so on.
voltage fall times. However, the switching current
These isolated gate drivers usually suffer from low
during turn-off is different.
drive strength, especially when the drive current
At the end of the switching event, the IGBT has a
capability is below the 2A range. Traditionally, these
“tail current” that does not exist for the MOSFET.
drive applications use discrete circuits to boost the
This tail is caused by minority carriers trapped in
drive current. Recently, there have been several
the “base” of the bipolar output section of the IGBT.
gate driver ICs developed to replace the discrete
This causes the IGBT to remain turned on. Unlike
solution. Figure 4 illustrates this trend.
a bipolar transistor, it is not possible to extract
In order to take advantage of the low conduction
these carriers to speed up switching, as there is no external connection to the base section. Therefore,
losses in IGBTs, gate drivers need to operate at
the device remains turned on until the carriers
voltages much higher than their threshold voltage
recombine. This tail current increases the turn-off
in the range of 15-18V. Furthermore, an IGBT is a
loss which requires an increase in the dead time
minority-carrier device with high input impedance
between the conduction of two devices for a given
and large bipolar current-carrying capability. The switching characteristics of an IGBT are similar to
phase of a half-bridge circuit.
that of a power MOSFET. For a given condition
Having a negative voltage (–5V to –10V) at the
when turned on, the IGBT behaves much like to a
gate helps to reduce the turn-off time by helping to recombine the trapped carriers. When the IGBT is
Power electronics in motor drives: Where is it?
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June 2015
Gate Driver
Gate Driver
Gate Driver
M
Microcontroller
Gate Driver
Gate Driver
1 2 Microcontroller
Isolated Gate Driver
3
Gate Driver 4 IC for Current Boost 5
Gate Driver
C
Figure 4. Three-phase inverter topology with boost gate driver
power supplies for IGBT gate drivers in the power stage turned on the high dv/dt and parasitic capacitance
efficiency. More details and construction of this
between gate and emitter generates voltage spikes
topology with a push pull converter can be found in
across the gate terminal. These spikes can cause a
the TI design guide [2].
false turn-on of the bottom IGBT. Having a negative
Other power supplies
voltage at the gate helps to avoid this false turn-on
Figure 3 shows an offline power supply that draws
trigger. Usually 15V to 18V is applied to the gate to
power from the three-phase universal AC line to
turn-on the device and a negative voltage of –5V to
a regulated 24V DC output. Because of the low-
–8V is applied to turn off the IGBT. This requirement
power level (below 75W), power factor correction
is key to determine the power supply rating to the
(PFC) is not needed. These offline power supplies
IGBT driver.
are typically fly-back topology converter ICs that
Typically, such a power supply is a PWM controller
could be a controller with external MOSFET, or
with a topology that has the ability to scale the
an integrated MOSFET controller or switcher. The
output power while driving these high-power IGBTs.
choice of the power supply IC is flexible and is
Typical inputs for these power supplies are regulated
influenced by the power level, number of outputs,
to 24V (to be explained shortly). One example of a
and accuracy of the regulation. This offline power
classic topology used for this power supply is the
supply is usually a separate module.
push-pull isolated converter. This topology is similar
The 24V DC output is the system power bus in
to a forward converter with two primary winding.
the AC motor drive system that is input into the
The advantage that push-pull converters have over
bias power supply for the power stage and non-
fly-back and forward converters is that they can be
isolated DC/DC converter. This non-isolated DC/DC
scaled up to higher powers, in addition to higher
Power electronics in motor drives: Where is it?
regulator from the 24V system provides power
6
June 2015
to the controller, communications and safety
48-600V, depending on the power levels. The
microcontrollers, interface transceivers, and data
switch is usually a power MOSFET switching at
converters.
around 100 kHz. Gate drivers are high-side, lowside or half-bridge drivers per inverter phase with
BLDC motor drives
no isolation requirement. Protection features are
The brushless DC (BLDC) is on trend for becoming
not as critical as those needed for the AC motor
the most popular choice, replacing brushed DC and
drive, except for dead-time control to avoid shoot-
AC motors in markets such as HVAC, especially for
through since the high-side and low-side drivers are
its higher efficiency and high reliability. Of particular
operating from one IC.
interest are power tools and household appliances such as refrigerators, air-conditioners, vacuum
Power supplies
cleaners and other such white goods. Using BLDC
Bias power to the controller and gate drivers comes
in these market spaces lowers the system’s overall
off a regulated power supply from the battery. A
weight.
typical battery used in this space is the 18V nominal
Figure 5 shows a block diagram of the BLDC motor
Lithium-Ion (Li-Ion) five-cell battery. Since these are cordless tools, a wall charger is required to charge
drive in a cordless (battery-powered) power tool
the drill periodically. Typically, charging in the range
such as an electric drill. Power blocks are shown in
of 50–1000W is done using an isolated controller
blue.
that is topology-specific, depending on the power
Power stage
level. Also, PFC is generally not needed unless
A BLDC power stage is also an inverter similar to an
the power level is in the few hundred W. Typical
AC motor drive, except that the input can be single-
charging controllers are based off of a fly-back,
or three-phases. DC-rail voltages are typically
Li-Ion/ Li-Poly/ Ni-Cd/ Ni-MH Battery
Cell Protection
Fuel Gauge
Surge Protection
Battery Pack
interleaved fly-back, or push-pull topologies.
Charger
AC
MOSFET
AC/DC Charger
Current/ Power Protection (LM5060)
- +
8..265 V AC DC/DC
Interface Protect
Power Tool
Motor Driver (DRV8301)
CAN
Controller
*Optional for robotic or high-end devices
MOSFETs HS-LS/ Half Bridge Gate Criver Current Feedback
Op Amp
Position Feedback
Hall Sensor
Figure 5. Block diagram of a cordless BLDC motor drive
Power electronics in motor drives: Where is it?
7
June 2015
Summary Motor drives are becoming more efficient as power
References 1. Energy-Efficiency Policy Opportunities for Electric Motor-
electronic devices such as power switches (IGBTs
Driven Systems, International Energy Agency, 2011
and MOSFETs), gate drivers and bias supplies are
2. Isolated IGBT gate-drive push-pull power supply with 4
being incorporated. We discussed two key and popular motor drive systems: AC and BLDC, and covered the functionalities and role of gate drive circuits and associated bias supplies. Key areas such as isolation, voltage levels and protection features were highlighted. TI has several gate drivers for both drives, the ISO5500 and UCC27531 for AC
outputs (TIDA-00181), Texas Instruments 3. Download these datasheets: CSD19534Q5A, CSD19501KCS, DRV8301, DRV8302, DRV8303, DRV8308, ISO5500, LM5023, LM5030, LM5105, LM5109, UCC27211A, UCC27201A, UCC27531, UCC28710, UCC28910
motor drives. For brushless DC drives using the three-phase pre-drivers, TI offers the DRV8301/2/3/8, or single-phase, high-side/low-side drivers such as the UCC272xx and LM510x families. To drive 100V power MOSFETs such as the CSD19534Q5A or 80V CSD19501KCS, TI offers the UCC27211A/ UCC27201A, LM5105, LM5109 and the UCC272201A-Q1 (for automotive applications). Regardless of drives, TI offers a variety of PWM controllers for bias supplies such as UCC287xx, UCC28910, LM5030 and LM5023.
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